Chained recompression has unpredictable behavior and is not useful in practice. First, systems usually configure just one alternative recompression algorithm, which has slower compression/decompression but better compression ratio. A single alternative algorithm doesn't need chaining. Second, even with multiple recompression algorithms, chained recompression is suboptimal. If a lower priority algorithm succeeds, the page is never attempted with a higher priority algorithm, leading to worse memory savings. If a lower priority algorithm fails, the page is still attempted with a higher priority algorithm, wasting resources on the failed lower priority attempt. In either case, the system would be better off targeting a specific priority directly. Chained recompression also significantly complicates the code. Remove it. Signed-off-by: Sergey Senozhatsky --- Documentation/admin-guide/blockdev/zram.rst | 9 --- drivers/block/zram/zram_drv.c | 83 ++++++--------------- 2 files changed, 24 insertions(+), 68 deletions(-) diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst index 967b58c3aad2..60b07a7e30cd 100644 --- a/Documentation/admin-guide/blockdev/zram.rst +++ b/Documentation/admin-guide/blockdev/zram.rst @@ -533,15 +533,6 @@ unexpected results when the same algorithm is configured with different priorities (e.g. different parameters). `priority` is the only way to guarantee that the expected algorithm will be used. -During re-compression for every page, that matches re-compression criteria, -ZRAM iterates the list of registered alternative compression algorithms in -order of their priorities. ZRAM stops either when re-compression was -successful (re-compressed object is smaller in size than the original one) -and matches re-compression criteria (e.g. size threshold) or when there are -no secondary algorithms left to try. If none of the secondary algorithms can -successfully re-compressed the page such a page is marked as incompressible, -so ZRAM will not attempt to re-compress it in the future. - memory tracking =============== diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index 118b0b277e37..ff1931e700c3 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -2331,7 +2331,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, #define RECOMPRESS_IDLE (1 << 0) #define RECOMPRESS_HUGE (1 << 1) -static int scan_slots_for_recompress(struct zram *zram, u32 mode, u32 prio_max, +static int scan_slots_for_recompress(struct zram *zram, u32 mode, u32 prio, struct zram_pp_ctl *ctl) { unsigned long nr_pages = zram->disksize >> PAGE_SHIFT; @@ -2357,8 +2357,8 @@ static int scan_slots_for_recompress(struct zram *zram, u32 mode, u32 prio_max, test_slot_flag(zram, index, ZRAM_INCOMPRESSIBLE)) goto next; - /* Already compressed with same of higher priority */ - if (get_slot_comp_priority(zram, index) + 1 >= prio_max) + /* Already compressed with same or higher priority */ + if (get_slot_comp_priority(zram, index) >= prio) goto next; ok = place_pp_slot(zram, ctl, index); @@ -2391,8 +2391,7 @@ static bool highest_priority_algorithm(struct zram *zram, u32 prio) * Corresponding ZRAM slot should be locked. */ static int recompress_slot(struct zram *zram, u32 index, struct page *page, - u64 *num_recomp_pages, u32 threshold, u32 prio, - u32 prio_max) + u64 *num_recomp_pages, u32 threshold, u32 prio) { struct zcomp_strm *zstrm = NULL; unsigned long handle_old; @@ -2404,6 +2403,9 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page, void *src; int ret = 0; + if (!zram->comps[prio]) + return -EINVAL; + handle_old = get_slot_handle(zram, index); if (!handle_old) return -EINVAL; @@ -2426,51 +2428,10 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page, */ clear_slot_flag(zram, index, ZRAM_IDLE); - class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old); - - prio = max(prio, get_slot_comp_priority(zram, index) + 1); - /* - * Recompression slots scan should not select slots that are - * already compressed with a higher priority algorithm, but - * just in case - */ - if (prio >= prio_max) - return 0; - - /* - * Iterate the secondary comp algorithms list (in order of priority) - * and try to recompress the page. - */ - for (; prio < prio_max; prio++) { - if (!zram->comps[prio]) - continue; - - zstrm = zcomp_stream_get(zram->comps[prio]); - src = kmap_local_page(page); - ret = zcomp_compress(zram->comps[prio], zstrm, - src, &comp_len_new); - kunmap_local(src); - - if (ret) { - zcomp_stream_put(zstrm); - zstrm = NULL; - break; - } - - class_index_new = zs_lookup_class_index(zram->mem_pool, - comp_len_new); - - /* Continue until we make progress */ - if (class_index_new >= class_index_old || - (threshold && comp_len_new >= threshold)) { - zcomp_stream_put(zstrm); - zstrm = NULL; - continue; - } - - /* Recompression was successful so break out */ - break; - } + zstrm = zcomp_stream_get(zram->comps[prio]); + src = kmap_local_page(page); + ret = zcomp_compress(zram->comps[prio], zstrm, src, &comp_len_new); + kunmap_local(src); /* * Decrement the limit (if set) on pages we can recompress, even @@ -2481,11 +2442,18 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page, if (*num_recomp_pages) *num_recomp_pages -= 1; - /* Compression error */ - if (ret) + if (ret) { + zcomp_stream_put(zstrm); return ret; + } + + class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old); + class_index_new = zs_lookup_class_index(zram->mem_pool, comp_len_new); + + if (class_index_new >= class_index_old || + (threshold && comp_len_new >= threshold)) { + zcomp_stream_put(zstrm); - if (!zstrm) { /* * Secondary algorithms failed to re-compress the page * in a way that would save memory. @@ -2535,11 +2503,11 @@ static ssize_t recompress_store(struct device *dev, struct device_attribute *attr, const char *buf, size_t len) { - u32 prio = ZRAM_SECONDARY_COMP, prio_max = ZRAM_MAX_COMPS; struct zram *zram = dev_to_zram(dev); char *args, *param, *val, *algo = NULL; u64 num_recomp_pages = ULLONG_MAX; struct zram_pp_ctl *ctl = NULL; + u32 prio = ZRAM_SECONDARY_COMP; struct zram_pp_slot *pps; u32 mode = 0, threshold = 0; struct page *page = NULL; @@ -2604,7 +2572,6 @@ static ssize_t recompress_store(struct device *dev, * "algorithm" name lookup is ambiguous. */ algo = NULL; - prio_max = min(prio + 1, ZRAM_MAX_COMPS); continue; } } @@ -2624,7 +2591,6 @@ static ssize_t recompress_store(struct device *dev, continue; if (!strcmp(zram->comp_algs[prio], algo)) { - prio_max = min(prio + 1, ZRAM_MAX_COMPS); found = true; break; } @@ -2653,7 +2619,7 @@ static ssize_t recompress_store(struct device *dev, goto out; } - scan_slots_for_recompress(zram, mode, prio_max, ctl); + scan_slots_for_recompress(zram, mode, prio, ctl); ret = len; while ((pps = select_pp_slot(ctl))) { @@ -2667,8 +2633,7 @@ static ssize_t recompress_store(struct device *dev, goto next; err = recompress_slot(zram, pps->index, page, - &num_recomp_pages, threshold, - prio, prio_max); + &num_recomp_pages, threshold, prio); next: slot_unlock(zram, pps->index); release_pp_slot(zram, pps); -- 2.53.0.473.g4a7958ca14-goog